This invention pertains to microlithography, which is a key technology used in the manufacture of semiconductor integrated circuits, displays, and the like. More specifically, this invention pertains to microlithography performed using a charged particle beam such as an electron beam or ion beam.
The following discussion of conventional microlithography is set forth in the context of using an electron beam as a representative charged particle beam.
Pattern transfer using electron-beam drawing is highly precise, but has the fault of low throughput. Considerable research effort has been expended in investigating various technical approaches to improving throughput. These approaches include partial-pattern, single-shot exposure techniques referred to as cell projection, character projection, or block exposure.
In partial-pattern, single-shot techniques, certain repetitive portions of the circuit defined on a reticle are exposed many times onto respective regions of a die on the wafer (typically, these repetitive regions measure approximately 5 xcexcm-square on the wafer). Each different repetitive portion is defined by at least one respective region on the reticle. This technique is used especially whenever the reticle pattern has large numbers of identical features having the same configuration, such as in a memory chip. Unfortunately, certain portions of the circuit that are not repeated must be transferred to the wafer using another technique, such as a variable-shaped-beam technique. Having to employ two or more different microlithography techniques to expose each die on the wafer results in a throughput that often is too low for use in mass production of wafers. Also, the throughput obtainable using the variable-shaped-beam technique is low.
To improve throughput, electron-beam reduction-transfer microlithography techniques have been devised. In certain apparatus employing such a technique, the reticle has a field defining the pattern for an entire die to be transferred to the wafer. The reticle field is illuminated using an electron irradiation beam, and an image of the illuminated field is xe2x80x9creducedxe2x80x9d (demagnified) and transferred onto the wafer by a projection-optical system. Whereas this full-field transfer technique offers prospects of vastly higher throughput than partial-pattern single-shot techniques, aberrations over a large field currently cannot be controlled satisfactorily to achieve the desired resolution. Also, reticles suitable for transferring an entire field in one shot are extremely difficult to fabricate.
In view of the above, electron-beam microlithography techniques currently receiving the most attention are those in which the reticle pattern is divided into a large number of field portions, termed xe2x80x9csubfields,xe2x80x9d that are exposed individually and sequentially onto the wafer by the electron beam. Such techniques are termed xe2x80x9cdivided-reticlexe2x80x9d microlithography methods. The subfields are exposed and transferred using a projection-optical system having a large optical field. Each subfield is exposed using a respective xe2x80x9cshot.xe2x80x9d To expose a subfield, the illumination beam is directed to the desired subfield so as to transfer an image of the subfield to a respective position on the wafer. As the subfield is being exposed, aberrations such as image defocusing and field distortion can be corrected in real time. On the wafer, the images of individual subfields are situated such that they are xe2x80x9cstitched togetherxe2x80x9d (placed contiguously relative to each other) to form the complete pattern. Divided-reticle exposure achieves better resolution and transfer accuracy over a wider optical field than achievable using a single-shot transfer of an entire die. Exploiting the wide exposure field in a divided-reticle exposure apparatus, high throughput can be achieved by continuously moving the respective stages on which the reticle and wafer are mounted.
In a cell-projection apparatus, the reticle is stationary during exposure, but the wafer is moved continuously. Under such conditions, the reticle image must be displaced smoothly using a deflector so that the reticle image tracks the motion of the wafer. Whenever an image is displaced using a deflector, the magnitude of beam deflection correspondingly changes, which causes continuous changes in the magnitude of deflection distortion. Heretofore, this change in deflection distortion was relatively slight and exhibited virtually no effect on exposure-position errors and image-defocusing. As a result, the amount of correction applied to correct deflection distortion was not revised (updated) continuously according to changes in the magnitude of deflection.
However, recent RandD has been aimed at expanding the deflection range and increasing the velocity of stage motion so as to further increase throughput. Deflection distortion generally is proportional to the cube of the corresponding magnitude of beam deflection. Consequently, as the magnitude of beam deflection increases, the magnitude of change in deflection distortion during a shot can reach a level that no longer can be ignored. Also, as circuit patterns continue to increase in density and complexity, the required tolerances for exposure accuracy and image defocusing become increasingly stringent. Therefore, it now is necessary to consider changes in aberrations, previously regarded as negligible, that accompany changes in the deflection magnitude during each shot.
In divided-reticle microlithography apparatus, the respective image of each illuminated subfield of the reticle typically is reduced (demagnified) as projected onto the wafer. Projection typically is performed using symmetrical magnetic doublet (SMD) electron-lens systems. Projection and exposure are performed while continuously and synchronously moving the reticle stage and the wafer stage. Demagnification is according to a xe2x80x9cdemagnification ratio,xe2x80x9d which is a factor by which an image as formed on the wafer is smaller than the corresponding region on the reticle. During exposure, the velocity of the wafer stage to the velocity of the reticle stage nominally is equal (but see below) to the demagnification ratio of the projection-optical system. Even under such conditions, the magnitude of deflection of the reticle image relative to the projection-optical system will change during each shot. I.e., the trajectory of the imaging beam from the illuminated region of the reticle to the imaging position on the wafer changes during each shot.
In actual practice, in a divided reticle, individual subfields typically are separated from one another by struts. The struts provide substantial rigidity and mechanical strength to the reticle, and serve to conduct heat away from the reticle during illumination of the reticle. The struts normally are configured in a grid pattern, with individual subfields being located in respective spaces between adjacent struts. These aspects will be described later below with reference to FIGS. 2(A)-2(C). Respective images of the struts are not projected onto the wafer. Consequently, the ratio of movement velocity of the substrate to the movement velocity of the reticle is not exactly equal to the demagnification ratio. I.e., the reticle moves slightly faster than indicated by the ratio, as described below with reference to FIG. 3.
Because of the higher velocity of the reticle during exposure, it is necessary to change the position of the projected image as xe2x80x9cseenxe2x80x9d from the projection-optical system during each shot. In other words, it is necessary to continuously change the magnitude of positional change from the viewpoint of the optical-lens column (i.e., the amount of movement in the image point due to movement of the object point of the projection lens), as well as to continuously change the magnitude of beam deflection to compensate for the velocity differential.
Ideal positioning of the reticle stage, on which the reticle is mounted, is not limited to microlithographic exposure systems that transfer a continuously moving reticle. In conventional scanning optical steppers, the reticle stage has a two-tier structure, in which a fine-movement stage (configured to move within a small but highly accurate range) is situated atop a high-speed stage (configured to move at high velocity over a wide range). The positional accuracy of the high-speed stage is relatively xe2x80x9ccoarse,xe2x80x9d but any positional error of the high-speed stage is compensated for by the fine-movement stage. Unfortunately, the two-tier reticle-stage configuration is costly, and also results in excessive overall thickness of the reticle stage.
Wafer stages having a two-tier configuration also have the same disadvantages.
In view of the shortcomings of the prior art as summarized above, an object of the invention is to provide charged-particle-beam (CPB) exposure apparatus and methods providing improved accuracy of pattern transfer.
To such end, and according to a first aspect of the invention, methods are provided for performing a microlithographic transfer of a pattern onto a sensitive exposure surface of a substrate using a charged particle beam. In one embodiment of such methods, the substrate (mounted on a substrate stage) is exposed with multiple successive shots of a charged-particle patterned beam onto respective specific locations on the exposure surface while moving the substrate stage and deflecting the patterned beam to the respective specific locations. Thus, the exposure surface is imprinted with the pattern. Meanwhile, during the shots, respective amounts of aberration correction are applied (typically by deflecting the patterned beam) as the patterned beam is deflected to the respective specific locations. In a given shot in which deflection of the patterned beam has changed from a previous position, the amount of the correction is updated so as to apply an appropriate aberration correction for the given shot. The amount of aberration correction applied in a shot typically corresponds to the respective specific location of the patterned beam. The aberration correction is directed to one or more aberrations such as deflection-position distortion, defocusing, astigmatism, image rotation, image magnification shift, and image astigmatic distortion. For example, deflection-position distortion can be corrected using a deflector; defocusing, image magnification shift, and image rotation can be corrected using a focusing coil; and astigmatism and astigmatic distortion can be corrected using a stigmator or astigmatic corrector.
In another method embodiment, the illumination beam is deflected to an irradiation area of the reticle so as to illuminate a respective pattern portion in the irradiation area. The patterned beam is projected and deflected to a respective location on the exposure surface so as to form an image at the location of the respective pattern portion illuminated in the irradiation area. These steps are repeated as required to perform, by multiple successive shots, transfer of images of respective pattern portions (illuminated in respective irradiation areas) to respective locations on the exposure surface. As in the first embodiment, during the shots, respective amounts of aberration correction are applied (typically by deflecting the patterned beam) as the patterned beam is deflected to the respective specific locations. Accompanying progression to a given shot, the amount of aberration correction is updated so as to apply an appropriate aberration correction for the given shot.
In another method embodiment, the substrate is mounted on a movable substrate stage and the reticle is mounted on a movable reticle stage. During exposure, the reticle stage is moved at a reticle-stage velocity while the illumination beam is deflected to an irradiation area on the reticle so as to illuminate a respective pattern portion in the irradiation area. Meanwhile, the substrate stage is moved at a substrate-stage velocity while the patterned beam is projected to a respective location on the exposure surface so as to form at the location a demagnified image of the respective pattern portion illuminated in the irradiation area. These steps are repeated as required to perform, by multiple successive shots, transfer of images of respective pattern portions in respective irradiation areas, to respective locations on the exposure surface so as to stitch the images together and transfer the pattern to the exposure surface. If, during any of these exposures, the demagnification ratio of an image as projected onto a respective location on the exposure surface does not match a ratio of the substrate-stage velocity to the reticle-stage velocity, then the image is caused to track movement of the respective location on the exposure surface by appropriately deflecting the patterned beam. During the shots, as the patterned beam is deflected, respective amounts of aberration correction are applied (e.g., by deflection) to the patterned beam. Accompanying progression to a given shot, if the required amount of aberration correction has changed, then the amount of correction is updated so as to apply an appropriate aberration correction for the given shot.
In yet another method embodiment, the substrate and reticle are mounted on respective movable stages. The reticle stage is moved while directing an illumination beam to an irradiation area on the reticle so as to illuminate a respective pattern portion in the irradiation area. Meanwhile, the substrate stage is moved while projecting the patterned beam to a respective location on the exposure surface so as to form at the location a demagnified image of the respective pattern portion illuminated in the irradiation area. These steps are repeated as required to perform, by multiple successive shots, transfer of images of respective pattern portions in respective irradiation areas to respective locations on the exposure surface. While making the exposures, a deflection is imparted to the illumination beam (e.g., by a deflector located in the illumination-optical system) as required to correct an error in irradiation position of the illumination beam on the reticle relative to an ideal position of the irradiation area on the reticle. Also, a deflection is imparted (e.g., by a deflector located in the projection-optical system) to the patterned beam as required to correct an error in the location of an image, as projected onto the exposure surface, of a respective pattern portion (illuminated in the respective irradiation area) caused by a reticle-position error. This method can include one or more of the following steps: (a) correcting a deflection distortion arising from use of the illumination-beam deflector to deflect the illumination beam to correct a deviation of the reticle from an ideal reticle position, and (b) correcting a deflection distortion arising from use of the patterned-beam deflector to deflect the patterned beam to correct a position error of the projected image on the exposure surface as caused by a reticle-position error.
In this embodiment, whenever the reticle position has shifted from its ideal position as a result of the reticle stage not being positioned ideally, the illumination position can be corrected using an illumination-beam deflector so that the center of the illumination beam desirably is positioned substantially at the center of the exposure field of the reticle. If necessary or desired, deflection distortion imparted by the deflector used to move the illumination beam also can be corrected at this time. Thus, the exposure field on the reticle can be illuminated properly.
In addition, whenever the reticle position has shifted from its ideal position, the transfer position of the reticle image on the exposure surface also is shifted. This shift can be corrected by a deflector desirably disposed between the reticle and the substrate. If necessary or desired, correction of any deflection distortion imparted by this deflector can be made at this time. Thus, transfer and exposure can be performed at the correct location on the substrate even if the reticle position has shifted.
Furthermore, if the substrate has shifted from its ideal position, the transfer position on the exposure surface can be corrected by a deflector located between the reticle and the wafer. If necessary, correction can include correcting a deflection distortion imparted to the patterned beam by this deflector. Thus, transfer and exposure can be performed at the correct location on the substrate, even if the position of the substrate has shifted.
Only deflection distortion has been mentioned in the foregoing summary of this embodiment. Magnitudes of deflection can differ from corresponding ideal magnitudes as a result of either or both the reticle and substrate shifting from their respective ideal positions. Aberrations other than deflection distortion can arise from these positional differences or from differences in deflection magnitude (e.g., one or more of focus, astigmatism, image rotation, image-magnification shift, and image-astigmatic distortion). These aberrations can be corrected as necessary or desired using a deflector, a focus compensator, a stigmator, a rotation-correction coil, a magnification-correction coil, or the like.
In yet another method embodiment, the pattern on the reticle is defined in a manner in which the pattern is divided into multiple respective pattern segments. The substrate and reticle are mounted on respective movable stages. The illumination beam is provided with a transverse-profile area that is smaller than a pattern segment. While moving the reticle stage, the illumination beam is deflected in a scanning manner onto an irradiation area of the reticle so as to illuminate a respective pattern portion in the irradiation area. Meanwhile, while moving the substrate stage, the patterned beam is projected in a scanning manner to a respective location on the exposure surface so as to form at the location a demagnified image of the respective pattern portion illuminated in the irradiation area. These steps are repeated as required to perform, by multiple successive shots made by scanning the illumination and patterned beams, transfer of images of respective pattern portions in respective irradiation areas to respective locations on the exposure surface. During the shots, respective amounts of aberration correction are applied as the patterned beam is deflected to the respective specific locations. Accompanying progression to a given shot, the amount of aberration correction is updated as required so as to apply an appropriate aberration correction for the given shot.
In yet another method embodiment, the substrate and reticle are mounted on respective movable stages. While moving the reticle stage, the illumination beam is deflected to an irradiation area of the reticle so as to illuminate a respective pattern portion in the irradiation area. Meanwhile, while moving the substrate stage, the patterned beam is deflected and projected to a respective location on the exposure surface so as to form at the location a demagnified image of the respective pattern portion illuminated in the irradiation area. These steps are repeated as required to perform, by multiple successive shots, transfer of images of respective pattern portions to respective locations on the exposure surface. During a shot, the patterned beam is deflected in a manner by which the image of the respective pattern portion in the illuminated irradiation area tracks movement of the respective imaging location on the exposure surface. Also, during a shot, an amount of aberration correction is applied as the patterned beam is deflected to the respective specific locations. Accompanying progression to a given shot, the amount of aberration correction is updated as required so as to apply an appropriate aberration correction for the given shot. For the given shot, if a position of the reticle is not an ideal reticle position and a corresponding position of the substrate is not an ideal substrate position, then the patterned beam can be deflected further (e.g., using a deflector in the projection-optical system) to apply respective corrections to the reticle position and substrate position and to correct any deflection distortions of the patterned beam, so as to form the respective image at an ideal location on the exposure surface.
This embodiment combines the steps of correcting aberration changes that accompany changes in deflection during a shot and correcting aberrations arising from differences in deflection accompanying deviations from the ideal reticle and/or substrate positions.
According to another aspect of the invention, charged-particle-beam microlithography apparatus are provide for transferring a pattern to a sensitive exposure surface of a substrate. One embodiment of such an apparatus comprises a movable substrate stage for mounting the substrate for exposure. An illumination-optical system is situated and configured to direct an illumination beam to an irradiation area on the reticle. A projection-optical system is situated and configured to direct a patterned beam to a respective location on the exposure surface so as to form an image at the location of the respective pattern portion in the illuminated irradiation area. The apparatus also includes a central controller that is connected to the substrate stage, the illumination-optical system, and the projection-optical system. The central controller is configured to: (1) coordinate movements of the substrate stage and respective operations of the illumination-optical system and projection-optical system to expose the substrate with multiple successive shots of the patterned beam deflected onto respective locations on the exposure surface, so as to imprint the exposure surface with the pattern; (2) during a shot, apply an amount of aberration correction as the patterned beam is deflected to the respective locations; and (3) update the amount of the correction in a given shot in which deflection of the patterned beam has changed from a previous shot, so as to apply an appropriate aberration correction for the given shot.
In another embodiment of an apparatus according to the invention, the reticle and substrate are mounted on respective movable stages. The apparatus includes an illumination-optical system and projection-optical system as summarized above. The central controller, connected to the stages and the optical systems, is configured to: (1) coordinate respective movements of the stages and respective operations of the illumination-optical system and projection-optical system to expose the substrate with multiple successive shots of the patterned beam deflected onto respective locations on the exposure surface, so as to imprint the exposure surface with a stitched-together image of the pattern; (2) during a shot, apply an amount of aberration correction; and (3) if a required amount of aberration correction has changed with progression to a given shot, then update the amount of aberration correction so as to apply an appropriate aberration correction for the given shot.
Yet another apparatus embodiment includes stages and optical systems as summarized above, as well as a central controller configured to coordinate respective movements of the stages at respective stage velocities and respective operations of the illumination-optical system and projection-optical system to expose the substrate with multiple successive shots of the patterned beam deflected onto respective locations on the exposure surface, so as to imprint the exposure surface with a reduced, stitched-together image of the pattern. In a shot, depending upon the demagnification ratio of an image of the respective pattern portion (in the irradiation area), as projected onto the respective location on the exposure surface, the central controller causes the image of the respective pattern portion to track movement of the respective location on the exposure surface by appropriately deflecting the patterned beam. The central controller also applies respective amounts of aberration correction as the patterned beam is deflected to the respective specific locations. If a required amount of aberration correction has changed with progression to a given shot, then the central controller updates the amount of aberration correction so as to apply an appropriate aberration correction for the given shot.
In this embodiment, the pattern field of the reticle is divided into multiple small fields (e.g., subfields), and struts extend between the small fields to provide mechanical strength and rigidity to the reticle. The reticle stage and substrate stage typically are moved so that their respective stripe-exposure start points and stripe-exposure end points are synchronized with each other. For example, if the ratio of the movement velocity of the exposure surface relative to the movement velocity of the reticle does not match the demagnification ratio at which the reticle is imaged onto the exposure surface, then the patterned beam is deflected (desirably using a deflector included inside a projection lens) during exposure. Thus, the projected image is caused to track the motion of the corresponding region of the exposure surface.
Yet another apparatus embodiment comprises stages and optical systems as summarized above. Also, a central controller is provided that is configured to coordinate respective movements of the stages at respective stage velocities and to coordinate respective operations of the illumination-optical system and projection-optical system to expose the substrate with multiple successive shots of the patterned beam deflected onto respective locations on the exposure surface, so as to imprint the exposure surface with a reduced, stitched-together image of the pattern. In a shot, the central controller detects an error in irradiation position of the illumination beam on the reticle relative to an ideal position of the irradiation beam on the reticle, and deflects the illumination beam as required to correct the error. In the shot, the central controller also predicts an error in the location of an image, as projected onto the exposure surface, of the respective portion of the pattern illuminated in the irradiation area caused by the error in reticle position, and deflects the patterned beam as required to correct the error. The central controller can be configured further to (a) apply an amount of aberration correction as the patterned beam is deflected to the respective specific locations; and (b) if a required amount of aberration correction has changed with progression to a given shot, then update the amount of aberration correction so as to apply an appropriate aberration correction for the given shot.
Yet another apparatus embodiment includes stages and optical systems as summarized above. The apparatus utilizes a reticle in which the pattern is divided into multiple respective pattern segments. The illumination beam has a transverse-profile area that is smaller than a pattern segment. The apparatus also includes a central controller configured to: (1) coordinate respective movements of the stages at respective stage velocities and respective operations of the illumination-optical system and projection-optical system to expose the substrate with multiple successive scanned shots of the patterned beam deflected onto respective locations on the exposure surface, so as to imprint the exposure surface with a reduced, stitched-together image of the pattern; (2) apply an amount of aberration correction as the patterned beam is deflected in a scanning manner to the respective specific locations; and (3) if a required amount of aberration correction has changed with progression to a given shot, then update the amount of aberration correction so as to apply an appropriate aberration correction for the given shot.
Yet another apparatus embodiment includes stages and optical systems as summarized above. The apparatus also includes a central controller configured to coordinate respective movements of the stages at respective stage velocities, and to coordinate respective operations of the illumination-optical system and projection-optical system to expose the substrate with multiple successive shots of the patterned beam deflected onto respective locations on the exposure surface, so as to imprint the exposure surface with a reduced, stitched-together image of the pattern. In a shot, the central controller deflects the patterned beam to cause the respective image of the respective pattern portion illuminated in the respective irradiation area to track movement of the respective location on the exposure surface. The central controller also applies an amount of aberration correction as the patterned beam is deflected to the respective specific locations. If a required amount of aberration correction has changed with progression to a given shot, then the central controller updates the amount of aberration correction so as to apply an appropriate aberration correction for the given shot. For the given shot, the central controller further deflects the patterned beam if a position of the reticle is not an ideal reticle position and a corresponding position of the substrate is not an ideal substrate position, so as to apply respective corrections to the reticle position and substrate position, correct any deflection distortions of the patterned beam, and form the respective image at an ideal location on the exposure surface.
Yet another apparatus embodiment comprises a movable reticle stage on which a divided reticle is mounted for exposure. An illumination-optical system is situated and configured to irradiate an irradiation area, including a respective partial field of a reticle pattern, of the reticle with a charged-particle illumination beam. The substrate is mounted on a movable substrate stage for exposure. A projection-optical system is situated and configured to a direct and focus a charged-particle patterned beam, propagating downstream of the irradiation area on the reticle, to a respective location on the exposure surface, to form an image at the respective location of the respective partial field illuminated in the irradiation area. The projection-optical system comprises deflection-aberration-correction means, which comprises means for applying a correction to a continuously changing aberration produced from corresponding changes in a patterned-beam trajectory within the projection-optical system during exposure of the irradiation areas. The aberration-correction means also includes means for changing an amount of the correction as required or desired to update the correction as exposure proceeds.
The reticle used with this embodiment typically is divided into multiple small fields (xe2x80x9cpartial fieldsxe2x80x9d such as subfields), usually separated by struts. Groups of small fields are arrayed into stripes. During exposure, the reticle stage and substrate stage are moved so that their stripe-exposure start points and stripe-exposure end points are synchronized with each other. A deflector (desirably located inside a projection lens) is configured to cause the projected image of a small field to track the movement of the corresponding region on the exposure surface. Alternatively to step-wise exposure of the small fields, exposure can be performed by deflecting and scanning the illumination beam (desirably having a transverse dimension that is smaller than the scanned small field). Thus, a corresponding area on the exposure surface is exposed (by scanning the patterned beam) that corresponds with the small field. The dimensions of the small field are configured to allow such deflection-scanning of the illumination beam. For example, the horizontal width of the small field corresponds to a direction perpendicular to the stage-scanning direction. The small field has a horizontal width corresponding to the deflection width of the beam. As a result, an entire small field can be illuminated by deflecting and scanning the illumination beam parallel to the horizontal width of the small field. At the exposure surface, the patterned beam is scanned in a direction perpendicular to the direction of continuous movement of the substrate stage. Meanwhile, the patterned beam is deflected in the direction of continuous movement of the stage using a deflector (desirably disposed inside a projection lens).
In this embodiment, changes in aberrations (e.g., distortion) that change during a shot are corrected. Aberrations other than deflection distortion include one or more of focus, astigmatism, image rotation, image-magnification shift, and image defocusing. These aberrations can be corrected using a deflector, a focus compensator, a stigmator, a rotation-correction coil, a magnification-correction coil, an image-defocusing coil, etc.
Corrections during a shot can be made in various ways. One way is to impose beam blanking while the correction values are being modified, then canceling beam blanking after the output of a compensator (e.g., deflector) performing the corrections has stabilized. Another way is to smooth the output of the correcting compensator (e.g., deflector) and make corrections at the location of beam emission. For example, the beam-projection position is corrected by appropriately updating a distortion-correction value. Whenever such correction-value updating is not performed, the image-projection position may change slightly during the shot. This causes image-defocusing and deterioration of the projection position. This embodiment corrects these problems.
The foregoing and additional features and advantages of the invention will be more apparent from the following detailed description, which proceeds with reference to the accompanying drawings.